If the interface has changed, I wouldn't keep the old content because it would be confusing to your students. Also, if you're covering the same material with the new videos, the old lectures become redundant and less people will be inclined to finish the course. When I overhaul any of my courses, I generally let my students know well ahead of time as well as directly after I switch over the old lectures to the new ones via educational announcements and short video in the 1st section of the course, which has been helpful.
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